The Mozilla Foundation is considering dropping Firefox support for Mac OS X 10.5 Leopard in June 2012, when Firefox 13 is released. While 24% of current Firefox users run Leopard, the Mozilla Foundation feels that this number will drop to around 13% of users. Currently, only 14% of Firefox users run Mac OS X 10.7 Lion, and 53% run 10.6 Snow Leopard.
This is important, because most of the main threats Mac users face come via the Internet. The web browser is one of the most crucial tools in computer security, and if a browser is no longer supported for a specific platform, users will not be able to upgrade and take advantage of security updates. Given the high number of Firefox users running Leopard, this may be a problem if the numbers don't drop as much as the Mozilla Foundation's projections.
Apple has also stopped supporting Leopard, and Mac users running Leopard should at least consider upgrading to Snow Leopard, if possible, to ensure that they remain eligible for security updates. While many users running older operating systems do so because their computers cannot run newer versions of Mac OS X, those which can should be upgraded, at least for security reasons.